1. Field of the Invention
The present invention relates generally to radio links and more specifically to an apparatus for use in equipment providing a digital radio link between a fixed radio unit and a mobile radio unit.
2. Description of the Related Art
Equipment for providing such a radio link is described in Great Britain patent application No. 9304901.3. This application describes the use of Wiener-like filters for providing good estimates of the amplitudes of the inphase I and quadrature phase Q components of, for example, a spread spectrum pilot signal.
When demodulating DBPSK (Dual Binary Phase Shift Keying) with no pilot reference it is still possible to obtain and exploit a carrier reference by means of decision directed carrier extraction. Samples are fed into the Wiener filter and are modified according to the data decisions in order to provide a reference. Such a demodulator for performing this function is shown in FIG. 1, in which a plurality of Rake fingers are shown 10, 12, 14, 16. Each Rake finger has a signal correlator 18 which handles an inphase signal from a down-converter, and a further signal correlator 20 which handles a quadrature phase signal from a down-converter. The output signal from the correlator 18 is applied to a half linear multiplier 22 by way of a one bit delay circuit 26. The output of the half linear multiplier 22 is applied to an input of a Wiener-like filter 30. The output of the Wiener-like filter 30 is applied to a multiplier 34. The output of the correlator 18 is also applied to a further input of the multiplier 34, the output of which is applied to a first input of an adder circuit 38.
The output signal from the signal correlator 20 is applied to a half linear multiplier 24 by way of a one bit delay circuit 28. The output of the half linear multiplier 24 is applied to an input of a further Wiener-like filter 32. The output of the Wiener-like filter 32 is applied to an input of a multiplier 36. The output signal from the correlator 52 is also applied to a further input of the multiplier 36, the output of which is applied to a second input of the adder circuit 38. The output of the adder circuit 38 together with the output signals from the other Rake fingers 12, 14, 16 are applied to an adder circuit 40 which generates a combined sum of the input signals for application to a decision circuit 42. The decision circuit 42 merely identifies whether the signal is high or low and the output of the decision circuit 42 is fed back by way of a latch circuit 44 to a second input of the respective half linear multipliers 22, 24 in the Rake finger 10 and similarly to the half linear multipliers in the other Rake fingers 12, 14, 16. This is done to modify the signal applied to the respective Wiener filters 30, 32 in Rake finger 10 and similarly in the other Rake fingers. The output of the decision circuit 42 to applied to an input of a differential decode circuit 46 which is arranged to generate output data on an output line thereof.
The circuit shown in FIG. 1 uses hard decisions which are obtained using the sum over all the Rake fingers to remove the modulation from the received signal. The most up-to-date available decision is the previous one, so in order to remove the modulation this must be applied to the previous sample. This leads to a one sample delay. The delayed sample can be compensated by the previous decision. However, this must be used to provide a channel estimate for demodulation of the next sample. Thus, a one step predictor filter must be used. Inevitably this will mean that the performance will be inferior to systems incorporating a pilot reference which can apply symmetrical filtering.